PROJECT SUMMARY Bacterial pathogens use diverse strategies to evade host defenses and colonize mammalian tissues. Some bacteria evade aspects of the host immune system, such as neutrophils and antibodies, by hiding inside of macrophages. Macrophages, however, are good bacterial killers in their own right. Therefore, how bacteria withstand macrophage defenses and acquire sufficient nutrients to survive and even replicate remains a key question in infection biology. We intend to close this knowledge gap by developing new tools to probe macrophage-bacterial interactions. Some of these tools could become the basis of antibacterial treatments. We study the Gram-negative intracellular pathogen Salmonella enterica, which colonizes macrophages in a murine typhoid fever model. We recently developed a quantitative, phenotypic, high content, in- macrophage screen for small molecules that prevent or increase bacterial growth inside of macrophages. Based on our pilot screen, we anticipate identifying a total of approximately 118 small molecules that prevent and 5 that promote S. enterica replication within macrophages. We propose to analyze compound chemical structure, evaluate purity, and establish the Effective Concentration50 and characterize the effect of compounds on infection dynamics and cell morphology using in vivo imaging.